Ink jet printing is a non-impact printing process in which a digital signal produces droplets of ink on media substrates such as paper or transparency films. Ink jet is widely used in industrial and office applications. Thermal or bubble jet drop-on-demand ink jet printers are the generally accepted medium price printers for personal computers. These printers have been widely accepted due to their high print quality, low cost, relatively quiet operation, environmental safety and color graphic capability.
The use of aqueous based inks for ink jet printers is well-known. The prior art ink jet inks generally use anionic dyes which contain a plurality of sulfonate or carboxylate anionic groups for their high solubility in aqueous solution. While the use of the more preferred cationic dyes, which have very desirable color properties such as brilliant hues and high chroma, some of them being fluorescent, is also known, their use is quite limited due to their relatively low solubility in an aqueous vehicle. The solubility can be significantly improved by lowering the pH of the ink to about 2, but the acidic environment tends to cause degradation of the pen components by inducing corrosion on the metal parts.
The low solubility of the dyes in general tend to cause the following problems in the ink jet printing mechanism.
1) Crusting, a persistent problem associated with aqueous based inks, describes the tendency of a ink to crust over a period of time, eventually leading to plugging of the orifice in the printer mechanism from which droplets of ink are expelled. Crusting is primarily the crystallization of the solid ink components around the orifice in the printhead, causing partial or full blockage of the orifice, leading to misdirection of the ink drop or prevention of drop ejection. The crusting problem arises from the evaporation of the liquid vehicle and the consequent precipitation of the solid ink components, primarily the dyes. PA1 2) Kogation, unique to thermal ink jet printing, describes the extent of decomposition of the ink components on the resistors of the ink jet pen as a consequence of repeatedly heating to high temperatures during printing. The decomposition products on the resistors will affect the bubble formation which is used to propel the ink droplets toward the substrate. PA1 (a) an aqueous carrier medium, present in a major amount; PA1 (b) a cationic dye; and PA1 (c) a cationic polymer.
Attempts have been made to solve the crusting problem by adding hygroscopic agents to reduce the rate of water evaporation by their ability to pick up water vapor from the air. While some improvement has been realized with this approach, a total solution to this problem has not been achieved. Further, it appears that no methods are known to prevent crusting or kogation of the cationic dyes in mildly acidic to mildly basic aqueous ink compositions (pH 4 to 9). Accordingly, there is a need in the art to provide an ink composition for ink jet printing which permits the use of the more desirable cationic dyes without the attendant crusting and kogation problems noted above.